1. Field of the Invention
The present invention relates to a 2-cyanoacrylate-based adhesive composition which is excellent in storage stability and particularly excellent in adhesive force and impact resistance and which has a high hardening rate, and it also relates to a process for preparing the 2-cyanoacrylate-based adhesive composition.
2. Description of the Prior Art
A 2-cyanoacrylate has been used as an adhesive which can instantaneously harden with the aid of a trace amount of moisture on the surfaces of adherends, and it is suitable for the adhesion of rubbers, plastics, lumbers and the like.
However, the 2-cyanoacrylate-based adhesive is usually poor in impact resistance, and so it cannot be used in a site where impact strength is required. Hence, this kind of adhesive has often restrictively been used for temporary fixing.
In order to overcome these drawbacks, there have been investigated a method which comprises introducing a long-chain alkyl group into an ester moiety of the 2-cyanoacrylate, a method which comprises adding an acrylic rubber or acrylonitrile-butadiene rubber to the 2-cyanoacrylate, and a method which comprises adding a plasticizer such as dioctyl phthalate to the 2-cyanoacrylate to impart flexibility thereto and to thereby improve durability to impact.
Furthermore, another method has also been employed which comprises adding a compound such as acetic acid, butyric acid or trifluoroacetic acid to the 2-cyanoacrylate to remarkably improve adhesive strength and to thereby improve impact resistance.
In the above-mentioned method which comprises introducing the long-chain alkyl group into the ester moiety to obtain the flexible 2-cyanoacrylate, it is necessary that the distillation temperature be raised, which makes the manufacture of the adhesive difficult. In addition, anionic polymerization is slow owing to moisture, and so a role as the instantaneous adhesive cannot be played inconveniently.
Moreover, the above-mentioned method which comprises adding the acrylic rubber or acrylonitrile-butadiene rubber to the 2-cyanoacrylate is known to be very effective means as a technique for improving impact resistance, humidity resistance, heat resistance, cold resistance and the like of the 2-cyanoacrylate-based adhesive (Japanese Patent Application Laid-open No. 500132/1984). However, also in this case, the storage stability of the mixed 2-cyanoacrylate-based adhesive is poor, and so when it is sold, a strict lot control is necessary.
With regard to the above-mentioned method which comprises adding a plasticizer such as dioctyl phthalate, the effect of imparting some flexibility to the 2-cyanoacrylate-based adhesive has been confirmed, but this method has drawbacks such as the deterioration of adhesive strength and the decline of adhesive force with time owing to the migration of the plasticizer.
As disclosed in Japanese Patent Application Laid-open Nos. 111936/1977, 77145/1977, 78933/1977, 80336/1977, 90536/1977, 66980/1980, 99980/1980, 142068/1980, 5807/1981 and 147067/1984, methods which comprise adding an acidic compound have the effect of remarkably improving adhesive force, and as a technique for enhancing the durability of the 2-cyanoacrylate-based adhesive, these methods are extremely useful means.
In this case, however, since the compound to be added is acidic, the hardening rate of the 2-cyanoacrylate-based adhesive noticeably declines if an excess amount of the acidic compound is added. In consequence, a function as the 2-cyanoacrylate-based instantaneous adhesive outstandingly deteriorates. Conversely, if the amount of the acidic compound is such as not-to have a bad influence on the hardening rate, the sufficient improvement effect of the adhesive force cannot be obtained.
Thus, it has been desired for a long time to develop a 2-cyanoacrylate-based adhesive composition which can remarkably improve adhesive force and can improve impact resistance and durability without introducing a specific ester group, without adding a rubber, an elastomer or a plasticizer, and without lowering the hardening rate of the adhesive.